Oil well sites often occur in porous rock formations (eg. sub-sea formations) whereby oil and water (and often gas) are stored or retained in the pores of the rock (eg. between grains of granular rocks such as sandstone). Specifically, the water is retained in the pore spaces as a film between any oil and, for granular rocks, individual grains and has low mobility. This water is referred to in the art as "irreducible". In other words, it is held in the rock in the pore spaces through capillary forces and is not removed when oil is retrieved from the site.
It is known that an estimation of oil reserves in a rock site can be made if the amount of irreducible water can be determined. One technique for determining this amount is to calculate and/or measure the water's resistivity (ie. electrical resistivity) and from this the water saturation (and thence oil saturation) of the oil well can be determined. However, existing tests for determining irreducible water resistivity are highly prone to error.
For example, resistivity manual measurement techniques involve the retrieval of a granular rock sample and the direct measurement of resistivity on water extracted from the rock sample but such tests are effected by mud filtrate contamination (ie. mud filtrate is captured when the sample is retrieved and contaminates the irreducible water in the sample).
To overcome introduced inaccuracies, calculating methods have been evolved which are not effected by mud filtrate contamination. One preferred technique is referred to as the "sp method" (spontaneous potential log method) which is a calculative technique based upon the voltage effect between the drilling mud and the formation water in a porous rock. The sign of the voltage switches polarity depending on whether the drilling mud is more or less saline than the irreducible water. The SP method is, however, notoriously uncertain and is effected by clay minerals and other influences not related to salinity. It would be advantageous to provide a technique that is not effected by such variables.